As they say, measure twice, cut once.
Somewhere during the process I inverted the layers and we got a nice plate of aluminium that’s exactly backwards. Oops. Also, bolt holes for the motor were specified too small, and I provided incorrect thread pitch detail on the mating bolt holes. Another detail was it needing a bit of clearance around the drive shaft. We’ll do it again properly.
On a positive note, the Robbins top arrived. I spec’d Stayfast Canvas rather than PVC… looks good but will fit it after the more serious work is done.